1. Field of the Invention
The present invention relates to a photoreflective detector, such as a photoreflective photointerruptor, for detecting the existence and position of an object, the tilt direction and angle of the object, etc., and a method for producing such a photoreflective detector.
2. Description of the Related Art
FIG. 1A is a cross sectional view of a conventional photoreflective detector 10, such as a photoreflective photointerruptor, for detecting the tilt direction and angle of an object by the direction of the light reflected by the object. FIG. 1B is a plan view of the photoreflective detector 10.
As is shown in FIGS. 1A and 1B, the conventional photoreflective detector 10 includes a substrate 1, a light emitting element 2 provided on the substrate 1 by die- and wire-bonding, and four light receiving elements 3A through 3D provided on the substrate 1 also by die- and wire-bonding. The four light receiving elements 3A through 3D are located equidistant from each other in a circular pattern having the light emitting element 2 as the center of the circle. Light L emitted by the light emitting element 2 is collected by a lens 4 provided above the substrate 1 and incident on the object 5. The light L is then reflected by the object 5 to be reflected light L' and is collected again by the lens 4. The reflected light L' forms an image represented by the dashed line in FIG. 1B and is incident on the light receiving elements 3A through 3D. In FIG. 1B, the optical outputs from the light receiving elements 3A through 3D have an equal level to one another.
FIGS. 2A and 2B are respectively a cross sectional view and a plan view of the photoreflective detector 10, illustrating a light path obtained when the object 5 is tilted at .DELTA..theta.. As is shown in FIG. 2B, the position of the image formed by the reflected light L' on the substrate 1 changes in accordance with the tilt angle of the object 5. In the state shown in FIG. 2B, the optical outputs from the light receiving elements 3A through 3D have different levels from one another.
The difference between an output S.sub.A from the light receiving element 3A and an output S.sub.C from the light receiving element 3C can be evaluated with, for example, expression (1). EQU S=(S.sub.A -S.sub.C)/(S.sub.A +S.sub.C) (1)
The relationship between tilt angle .DELTA..theta. of the object 5 and detection factor S obtained above is shown in FIG. 3. Specifically, detection factor S changes linearly with respect to tilt angle .DELTA..theta. of the object 5 in a certain range of angles. Accordingly, tilt angle .DELTA..theta. can be detected by obtaining detection factor S from expression (1).
The above explanation concerns the light receiving elements 3A and 3C. The same operation can be performed using the optical outputs from the light receiving elements 3B and 3D to detect the tilt direction and angle of the object 5 two-dimensionally.
In the above-described conventional photoreflective detector 10, the light emitting element 2 and the light receiving elements 3A through 3D are located on the same substrate 1. In such a structure, light La emitted from side surfaces of the light emitting element 2 (FIGS. 1A and 2A) is directly incident on the light receiving elements 3A through 3D. Since the light receiving elements 3A through 3D each generate an output signal from the light La directly received and the light L' reflected by the object 5, the denominator (S.sub.A +S.sub.C) of expression (1) increases, thereby reducing detection factor S. This indicates that the S/N ratio, which is the ratio of signal component S obtained by the light L' reflected by the object 5 with respect to non-signal component (noise component) N generated by direct incidence of the light on the light receiving elements 3A through 3D, is reduced.
FIGS. 4A through 4C show another conventional photoreflective detector 20. FIG. 4A is a side view, FIG. 4B is a cross sectional view, and FIG. 4C is a plan view of the detector 20.
The photoreflective detector 20 includes a light emitting element 12 and a light receiving element 13 in separate chambers 11A and 11B provided in a package 11. Light from the light emitting element 12 is incident on an object 15, and the light reflected by the object 15 is incident on the light receiving element 13.
The chambers 11A and 11B are formed of a light-transmitting molding resin using the same or different molds. In either case, the chambers 11A and 11B are formed by clamping the molds. The package 11, a part of which is sandwiched between the chambers 11A and 11B is formed of a light-shielding resin to avoid light from the light emitting element 12 from being directly incident on the light receiving element 13.
The photoreflective detector 20, which includes the light emitting element 12 and the light receiving element 13 arranged horizontally, cannot be significantly reduced in size. Furthermore, since the light emitting element 12 and the light receiving element 13 need to be molded separately, the production steps increases.
As is shown in FIG. 4C, the light emitting element 12 and the light receiving element 13 are aligned along the X axis. In the case where the light receiving element 13 is divided into a plurality of areas (for example, four areas), the detection factor obtained by X-axis rotation of the object 15 is different from the detection factor obtained by Y-axis rotation of the object 15.
Such a phenomenon will be described with reference to FIGS. 5A and 5B.
FIG. 5A is a cross sectional view of the photoreflective detector 20, illustrating the change in light paths when the object 15 tilts by angle .psi. around the Y axis. FIG. 5B is a cross sectional view of the photoreflective detector 20, illustrating the change in light paths when the object 15 tilts by angle .psi. around the X axis.
In the case shown in FIG. 5A, when the object 15 is positioned parallel to the surface on which the light receiving element 13 is mounted, as represented by solid line, the light from the light emitting element 12 is incident on the object 15 at angle .theta..sub.1 and reflected by the object 15 also at angle .theta..sub.1 to be incident on the light receiving element 13. When the object 15 tilts at angle .psi. around the Y axis as represented by dashed line, the light from the light emitting element 12 is incident on and reflected by the object 15 at angle .theta..sub.2.
In the case shown in FIG. 5B, when the object 15 is positioned parallel to the surface on which the light receiving element 13 is mounted, as represented by solid line, the light from the light emitting element 12 is incident on and reflected by the object 15 at 90 degrees. When the object 15 tilts at angle .psi. around the X axis as represented by dashed line, the light from the light emitting element 12 is incident on and reflected by the object 15 at angle .theta..sub.3.
As appreciated from the above description, the position of the substrate 1 on which the light reflected by the object 15 is incident is different when the object 15 tilts around the Y axis from when the object 15 tilts around the X axis. As a result, the detection factor of the photoreflective detector 20 is different between these two cases. The structure shown in FIG. 1B including a plurality of light receiving elements 3A through 3D positioned around the light emitting element 2 at an equal interval solves such a problem associated with change in detection factor, but there still remains the above-described problem of reduction in S/N ratio caused by direct light incidence on the light receiving elements 3A through 3D from the light emitting element 2.